1. Field of the Invention
The present invention relates to a circuit for detecting the rise of ambient temperature, using a forward voltage drop developed across a diode.
2. Related Art
A temperature detecting circuit has been proposed in Japanese Patent Application Laid-Open No. H.6-332555 and is located, for example, near an output transistor that feeds a load current to an electrical load. This temperature detecting circuit senses that the output transistor has overheated if such overheating has taken place.
The above-cited temperature detecting circuit is shown in FIG. 7A, and comprises a diode D used for temperature detection, a P-channel MOS transistor T1 connected between a voltage source VD and a ground, another P-channel MOS transistor T2 connected between the voltage source VD and the anode of the diode D, a P-channel MOS transistor T3 connected in parallel with the MOS transistor T2 and in series with the diode D, a comparator CMP for comparing a reference voltage Vref from a reference voltage source K with a voltage at the anode of the diode D, a P-channel MOS transistor T4 for connecting or disconnecting a current path going from the voltage source VD to the MOS transistor T3 according to the output from the comparator CMP, and a resistor R2 and an N-channel MOS transistor T5 for inverting the level of the output from the comparator CMP and delivering the output from a terminal S. The cathode of the diode D is coupled to ground potential, or 0 V. A reference current is fed to the P-channel transistor T1 via a resistor R1. The P-channel MOS transistors T1 and T2 together form a current mirror circuit. Similarly, the MOS transistors T1 and T3 together constitute a current mirror circuit.
This temperature detecting circuit operates in the manner described below. A constant current Ia that is made larger than the reference current flowing through the MOS transistor T1 by a given factor is constantly furnished to the diode D via the MOS transistor T2. If the temperature of the output transistor is low, the ambient temperature of the diode D and the temperature of its junction portion are also low. At this time, a large forward voltage drop takes place. The voltage at the anode of the diode D is higher than the reference voltage Vref, so that the output of the comparator CMP is low. This causes the MOS transistor T4 to conduct. The MOS transistor T3 supplies a constant current Ib to the diode D, the current Ib being larger than the reference current by a given factor. Therefore, the current I.sub.D flowing through the diode D is the sum of the constant current Ia supplied by the MOS transistor T2 and the constant current Ib supplied by the MOS transistor T3, i.e., I.sub.D =Ia+Ib. The anode voltage of the diode D increases further.
Under this condition, if the temperature of the output transistor described above rises, the ambient temperature of the diode D and the temperature of the junction portion also rise, decreasing the forward voltage drop. If the anode voltage of the diode D becomes lower than the reference voltage Vref, the output from the comparator CMP goes high, thus biasing the MOS transistor T4 to cutoff. As a result, the current path going from the voltage source VD to the MOS transistor T3 is disconnected. The current I.sub.D flowing through the diode D decreases down to the constant current Ia supplied only by the MOS transistor T2. The anode voltage of the diode D decreases further. If the output from the comparator CMP goes high, the MOS transistor T5 is turned ON, and the circuit produces a low output signal at the terminal S to indicate overheating of the output transistor.
If the temperature of the diode D subsequently drops down to the temperature produced when the output from the comparator CMP went high, i.e., the temperature obtained when the overheating was detected, the anode voltage of the diode D remains lower than the reference voltage Vref, because the current I.sub.D flowing through the diode D has been decreased. When the temperature drops further and becomes lower than the temperature obtained when the overheating was detected, the output from the comparator CMP goes back to a low level. Then, the original state is restored. That is, the constant current Ia from the MOS transistor T2 and the constant current Ib from the MOS transistor T3 are supplied to the diode D. In this state, the overheating is no longer detected.
The prior art temperature-detecting circuit shown in FIG. 7A detects the rise of ambient temperature, by making use of a forward voltage drop across the diode D. The two MOS transistors T2 and T3 are connected in parallel to control a constant current supplied to the diode D. That is to say, the current path for the MOS transistor T3 is connected or disconnected by the MOS transistor T4 according to the output from the comparator CMP. This makes the current I.sub.D (=Ia) flowing through the diode D when overheating of the output transistor is detected smaller than the current I.sub.D (=Ia+Ib) flowing through the diode D when overheating thereof is not detected. By controlling the current in this way, a hysteresis width is established between the temperature at which overheating of the output transistor is detected and a restoring temperature at which the overheating thereof ceases to be detected.
In the prior art temperature-detecting circuit shown in FIG. 7A, however, when the MOS transistor T4 is driven into conduction, this transistor T4 forms a substantial resistance in the current path for the diode D. The temperature characteristic of this resistive component has made it impossible to accurately establish the hysteresis width in detecting the overheating of the output transistor.
More specifically, the hysteresis width Thys between the temperature at which the overheating of the output transistor is detected and the restoring temperature is given by the ratio of the current I.sub.D (=Ia+Ib) flowing through the diode D when the overheating thereof is not detected to the current I.sub.D (=Ia) flowing through the diode D when the overheating thereof is detected, as given by Eq. (1) below. EQU Thys=VT.times.In{(Ia+Ib)/Ia}!/.alpha. (1)
where VT is a constant determined by the Boltzmann's constant k, the absolute temperature T, and the electron charge q, as given by Eq. (2) below, and .alpha. is the temperature coefficient of the forward voltage drop across the diode D. At normal temperatures, VT is approximately 26 mV . EQU VT=k.times.T/q (2)
In the temperature-detecting circuit shown in FIG. 7A, even if the temperature characteristics of the two MOS transistors T2 and T3 acting as constant current supply devices cancel out each other, the ratio of the current Ib supplied to the diode D by the MOS transistor T3 to the current Ia supplied to the diode D by the MOS transistor T2 is varied due to the temperature characteristic of the resistive component of the MOS transistor T4. As a result, the value of the { } item of Eq. (1) above varies. This makes it very difficult to maintain the hysteresis width Thys constant.
The above-cited Japanese Patent Application Laid-Open No. H.6-332555 also discloses a temperature detecting circuit as shown in FIG. 7B. This temperature detecting circuit is similar to the temperature detecting circuit shown in FIG. 7A except that a resistor R3 is connected in the current path going from the voltage source VD to this temperature detecting circuit instead of the MOS transistors T3 and T4. When the output from the comparator makes a transition from a low level to a high level (i.e., the overheating is detected), the output MOS transistor T5 is turned ON. As a result, the voltage applied to the MOS transistors T1 and T2 is decreased to a value obtained by dividing the voltage VD of the voltage source by the two resistances R2 and R3. This reduces the current I.sub.D flowing into the diode D from the MOS transistor T2. The hysteresis width used for detection of the overheating is established because of the voltage control as described above.
Even in the conventional circuit shown in FIG. 7B, however, the current I.sub.D flowing through the diode D is varied due to the temperature characteristics of the resistors R2 and R3. In consequence, it has been impossible to establish the hysteresis width accurately.